Device and method for optically measuring the characteristics of a substance utilizing three wavelengths of light

ABSTRACT

A first embodiment of the invention includes a light source (1) lighting two parallel optical branches (b1, b2) controlled respectively by two optical shutters (4, 6), one comprising a transparent cell (3) containing a substance to be studied, and the other being used to divert the light from the source. The light coming successively from one and from the other branch is applied selectively to three color filters (F1-F3) filtering three wavelengths selected according to the substance to be studied. The successive luminous intensities which have crossed each of the filters are measured through three detectors (D1 to D3) and the measurings are combined by means of a control (0). Analogous measuring sequences may be achieved with an embodiment comprising three light sources and a single detector. The device can be used for determination of the pH value of a substance.

FIELD OF THE INVENTION

The present invention relates to a method and device for measuringoptically the characteristics of a substance, such as, for example, theabsorbance thereof.

BACKGROUND OF THE INVENTION

In the French patent FR-A-2,689,636 there is described a device formeasuring the true optical absorbance of a substance contained in acell. By means of optical switching means, light from a light sourcesuch as a halogen lamp whose frequency spectrum is known is transmittedsuccessively to three optical filters of determined respectivewavelengths. The first one corresponds to the isobestic point of thecolored substance where the absorbance of the basic fraction of thesubstance is equal to that of its acid fraction and thereforeindependent of the pH value. The second one is in a part of the spectrumwhere the substance reacts most to the variations of the parameter to bemeasured. The third one is in a part of the spectrum where theabsorbance of the substance substantially undergoes no variation. Thelight from each of the filters is so directed that it runs alternatelythrough the cell containing the substance to be analyzed and an opticalby-pass branch including for example a neutral filter, an opticalby-pass fiber, or a reference cell, etc. The intensity of the successiveemergent rays is measured and the various measurements are processed bya management and computing unit which determines optical characteristicsof the substance, such as the true absorbance, which are independent ofthe possible fluctuations of the light source.

The method used provides accurate and reliable results, but the deviceimplementing the method is relatively costly since it includes severaloptical switches whose unit price is relatively high.

SUMMARY OF THE INVENTION

The object of the invention is also to measure optical characteristicsof a reacting substance contained in a transparent cell, such as itstrue absorbance, but the embodiment thereof is more simple and lesscostly than the prior art. It includes at least a light source, a firstoptical branch and a second optical branch allowing selectively passageof the light through the cell and outside it, an optical system forforming rays crossing the first optical branch and the second opticalbranch and a selective optical filter from an array of three selectivefilters, the first being centered on a first wavelength corresponding tothe isobestic point of the reacting substance, the second one on awavelength in a part of the light spectrum where the substance is themost sensitive, and the third one in another part of the spectrum wherethe substance is the least sensitive, means for measuring the intensityof the light crossing the optical system, a controller, and selectionmeans controlled by said control set and an electric power pack.

The device is characterized by the selection means consisting of twooptical shutters located respectively in the first branch and in thesecond branch, and of electric switching means.

The controller includes for example a control processor, an acquisitionunit for acquiring the luminous intensity measuring signals and aninterface set for controlling the selection means.

A first embodiment includes a single light source lighting the twobranches and the optical system comprises by-pass means for directingtowards the three filters the light rays which have crossed the first orthe second branch, the measuring means include three detectors formeasuring the light rays which have crossed the three filters, theelectric means comprise elements for connecting intermittently thedetectors to the controller set and a switch for connectingintermittently the lamp to the power pack.

According to another embodiment, the device includes three lightsources, the optical system comprises by-pass means for directing thelight from the sources respectively through the three selective filtersand dividing means for applying the filtered light to the first and thesecond branches, the measuring means include, a single detector formeasuring the light coming from one or from the other branch, and theelectric switching means include elements for supplying selectively oneof the three light sources.

The second optical branch includes for example an optical fiberassociated with a neutral filter or a cell identical to the first oneand containing a neutral substance.

The measuring method according to the invention when using the firstembodiment of the invention includes achieving measuring cycles underthe control of the controller, each cycle comprising:

a first measuring stage for directing light having crossed the cellssuccessively through the three filters and measuring the lightintensities coming from the three filters;

a second measuring stage for directing light which has crossed thesecond optical branch successively through the three filters andmeasuring the light intensities coming from the three filters; and

combining of the intensity values measured by each detector respectivelyby the first and second stages for determining the characteristics ofthe reacting substance.

When using the second embodiment of the invention, each measuring cyclemay also comprise:

a measuring stage for directing the light which has crossed successivelythe three filters towards the cell containing the reacting substance andmeasuring respective light intensities,

a measuring stage for directing the light which has crossed successivelythe three filters towards the second optical branch and measuring therespective light intensities; and

a combining of the light intensity values coming from the light sourcefor determining the characteristics of the reacting substance.

Whatever the embodiment used, the invention includes only two opticalshutters, the sequence of the comparative measuring operations necessaryfor the acquisition of the measurements being controlled by electricswitches which are easily achieved. The cost and the reliability of theinvention are therefore significantly improved.

In order to improve further the measuring precision, the device may alsocomprise means for measuring the ambient temperature and the supplyvoltage of each lamp.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the invention will be clear fromreading the description hereafter of two embodiments given by way of nonlimitative example, with reference to the accompanying drawings inwhich:

FIG.1 diagrammatically shows a first embodiment of the invention with asingle source and a selective filtering achieved on the emergent light;

FIG.2 diagrammatically shows a second embodiment of the invention with aselective lighting of the cell by means of three different beams;

FIG.3 shows a flowchart example for the implementation of the embodimentof FIG. 1,

FIG.4 details the stage of acquisition of the measurements in theflowchart of FIG.3;

FIG.5 shows a flowchart example for the implementation of the embodimentof FIG.2; and

FIG.6 details the operations achieved for each lamp.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

It should first be reminded that the pH value of a solution iscalculated through the relation:

    pH-pKi+log x/(l-x)                                         (1),

where pKi is a constant and x is the basic fraction of the substance.This parameter x is related to the absorbance A of the substance by therelation

    x=A/(c·I·.English Pound.)                (2)

where c is the concentration, l is the length of the optical pathcrossed by the rays and .English Pound. is the extinction coefficient ofthe cell. The absorbance is expressed as a function of the incidentintensity Ii applied to the cell and of the intensity Ie emergingtherefrom, through the relation: ##EQU1##

The values of the absorbance A are subject to large fluctuations if Ieis measuring only, on account of the lamp instability. In fact, it iswell known that the characteristics of such a source vary with time. Thecolor temperature for example is likely to change because of variouscauses due to the lamp itself: progressive vaporization of the filament,aging of the casing, etc, and to the instability of the power supply.This results in a notable modification in the form of the frequencyspectrum of the source. It may for example be observed that the colortemperature of the source may decrease in time by more than 10%, whichleads to high ratio variations between the luminous intensities appliedto the various filters, and consequently to measuring errors.

On account of its structure and of its implementation procedure, theinvention precisely allows the characteristic variations in time of thesource to be disregarded.

The first embodiment includes a light source 1 such as a halogen lampwith a tungsten filament. The light coming from source 1 is subdividedby an optical separator 2 into two light pencils which are directed bymeans of optical fibers b1, b2 for example, the first one towards a mainoptical branch including a cell 3 containing a reacting substance whosecolor variations are to be measured, followed by a first optical shutter4, the second one towards a bypass optical branch to form a beam passingoutside cell 3. This bypass branch comprises for example a neutralfilter 5 whose transmittance is selected substantially equal to theaverage transmittance of cell 3, followed by a second optical shutter 6.The outputs of the two switches 4, 6 are connected to an optical node 7.

The light beam from the optical node 7, coming selectively from one orthe other branch, is also subdivided by an optical separator 8 intothree beams which are directed by three optical fibers b3, b4, b5respectively towards three color filters F1, F2, F3. These three filterspass respectively the wavelengths 494 nm, 600 nm and 730 nm for example.The first one, F1, corresponds to the point known as isobestic point ofthe colored substance where the absorbance of the basic fraction of thesubstance is equal to that of its acid fraction and thereforeindependent of the pH value. The second, F2, is that for which thecolored substance reacts most to the variations of the parameter to bemeasured. The third one, F3, corresponds to a wavelength for which theabsorbance of the colored substance undergoes no variation. Each one ofthe three filters F1, F2, F3 works for example with a neutral filter F1whose transmittance is selected so as to balance the luminousintensities passing through the three branches b3, b4, b5. The lighthaving crossed respectively the three color filters F1, F2, F3 isapplied to three photoelectric detectors D1, D2, D3. The signalsdelivered thereby are applied to three inputs of a multiplexer M.

The device is managed by a controller 9 including a control processor10, an acquisition unit 11 connected to the output of multiplexer M andan interface 12 for controlling the optical shutters 4, 6 andmultiplexer M. The device further comprises a power supply 13 such as aelectrical storage in the case of an autonomous working of the device,this power supply being connected to lamp 1 by means of a switch I1 alsocontrolled by interface 12. Preferably, the device also includes a heatprobe 14 arranged next to the elements of the device to measure theambient temperature, this probe being connected to an input ofmultiplexer M, as well as a voltmeter for measuring the voltagedelivered by power supply 13.

Shutter 4 or 6 and the associated element 3 or 5 may be easily invertedin each of the branches b1, b2.

The measuring method allows the possible luminous intensity fluctuationof the source to be disregarded by determining each absorbance valuefrom measurements achieved by a single detector.

As shown by the flowcharts in FIGS.3, 4, each measuring cycle firstcomprises switching on of lamp 1 by the closing of switch I, followed bya series of 8, 12 or 16 measurings called "offset" measurings duringwhich, shutters 4 and 6 are closed, the noise signals affectingdetectors D1,D2, D3 are measured in sequence through a sequentialcontrol of multiplexer M, these signals being acquired and digitized byacquisition set 11. When the noise signals have been measured, shutter 4is opened and a sequence of measurings of the light crossing cell 3 onthe main branch b1 and successively being filtered through filters F1,F2, F3 is performed. After opening shutter 6 and by performing ananalogous sequence, measurements of the incident light from the sourcewhich has crossed the neutral filter 5 of bypass branch b2, thensuccessively filters F1 to F3, are acquired thereafter. To complete theprevious measurings, an acquisition of the temperature measured by heatprobe 14 and of the voltage applied to lamp 1 and measured by element 15is preferably achieved.

The absorbances A1, A2 and A3 corresponding to the three wavelengths ofthe color filters F1 to F3 are calculated from the intensities of thelight passing through the main branch containing cell 3 and through thebypass branch containing neutral filter 5. By applying relation (2), itmay be shown that the value of the basic fraction x of the substancestudied is obtained through the following relation:

    x=k·(A2-A3)/(A1-A3)

where k is a constant, and that, on account of the comparative methodused, any degradation in time of the characteristics of lamp 1 istotally disregarded. Processor 10 may for example deduce therefrom thepH value of the analyzed substance.

The temperature and the effective voltage applied to lamp 1 arepreferably taken into account so as to minimize errors. In fact, it maybe noted that measuring errors are of the order of 4% when thetemperature rises from 20° C. to 60° C. To compensate these variations,processor 10 applies to the measurements a correction according to thetemperature variation, such as a linear correction for example.

In order to minimize measuring errors further, the voltage variations(due to the discharge of the accumulator in case of self-containedelectrical supply), which cause the luminous intensity of lamp 1 tovary, may also be taken into account. Accurate measurements to within 1%may be obtained by taking these variations in temperature and in voltageinto account.

The previous embodiment is suitable if the cell contains a coloringagent which is not substantially affected by being lit directly by lamp1 and by receiving thereby the entire light spectrum.

In the opposite case, it is preferable to use the embodiment of FIG.2.In this instance, the main branch 11 consisting of cell 3 and opticalshutter 4, and the bypass branch including for example a neutral fiberare connected to an optical separator 16 linked to an optical selectionmeans S with three branches b'3, b'4, b'5. Each one of them includes alamp, respectively L1, L2, L3, analogous to lamp 1 (FIG. 1), which areconnected to power supply 13 respectively by three switches I2, I3, I4controlled by interface circuit 12. The light of these lamps is filteredrespectively by the previous three color filters F1, F2, F3 which workfor example with a neutral filter F1. The light filtered by the threecolor filters is directed, via an optical node 17, towards the input ofthe optical separator 16. The light coming from the two branches b1, b2is applied to a single photoelectric detector D. A heat probe 14 is alsoconnected to the input of acquisition unit 11 by means of a switch I5controlled by interface set 12. Similarly, a voltmeter 15 for measuringthe voltage applied to lamps L1 to L3 is interposed between the lampsand power supply 13, the signal of the voltmeter being applied toacquisition unit 11 by means of a switch I6.

With this embodiment, each measuring cycle comprises (FIGS.5, 6) threeidentical sequences implementing successively lamps L1, L2 and L3. Eachof the successive sequences comprises the switching on of thecorresponding lamp, L1 for the first one for example, and, shutters 4and 6 being closed, the signal delivered by the single photoelectricdetector D in the absence of light is acquired (offset voltage). Then,during a first opening time interval of shutter 4, several successivemeasurings of the signal of detector D are performed. The previousoperation is repeated for lamps L2 and L3 successively. Each sequenceends with an acquisition of the voltage measured by volmeter 15. Afterthe three previous sequences, each cycle is ended by an acquisition ofthe ambient temperature measured by heat probe 14. As previously, theluminous intensity measurements are combined to obtain the absorbancevalues in cell 3 which is for example deduced therefrom.

The various optical branches or paths allowing the light to be directedthrough cell 3, neutral filter 5 and the color filters may consist ofoptical fibers, but they may also be obtained with conventional opticalconvergence or divergence means ("overhead" optics).

Without departing from the scope of the invention, the neutral filterused for forming the reference medium may be replaced by a cellanalogous to cell 3 but containing a neutral substance so selected thatthe absorbance of this parallel cell is equal to the average absorbanceof the main cell.

We claim:
 1. A device for optically measuring modifications in areacting substance contained in a transparent cell, comprising:a singlelight source provided with an electric supply voltage and a specifiedlight spectrum, a first optical circuit, a first optical shutterarranged in said first optical circuit, a second optical circuit, asecond optical shutter arranged in said second optical circuit, anoptical diverter in said optical circuits for diverting incident lightfrom the single light source through the cell and through a referencemedium to an optical node, an optical separator for directing light fromthe optical node to three other optical circuits, a set of threeselective filters arranged respectively in said three other opticalcircuits, a first selective filter from the set being centered on afirst wavelength corresponding to an isobestic point of the reactingsubstance, a second selective filter from said set being centered on awavelength in a part of the light spectrum where the reacting substanceis the most sensitive and a third selective filter from the set beingcentered on another part of the light spectrum where the reactingsubstance is the least sensitive, a measuring means for respectivelymeasuring the light emanating from said three other optical circuits,including a set of three detectors for respectively detecting lightpassed by each of the set of three filters and producing output signalsrepresenting the detected light, an electric power supply for providingthe electrical supply voltage, a controller and an electric switchingmeans controlled by the controller for connecting intermittently saidthree detectors to said controller, for connecting the single lightsource to the power supply, and for selectively switching said opticalshutters.
 2. A device in accordance with claim 1 wherein the controllerincludes a controller microprocessor, an acquisition unit for acquiringthe output signals and an interface for controlling the selection means.3. A device in accordance with claim 1 further comprising means formeasuring an ambient temperature and producing a signal representing theambient temperature coupled to the controller.
 4. A device in accordancewith claim 1 further comprising means for measuring a supply voltage ofeach lamp and producing a signal representing the supply voltage of eachlamp coupled to the controller.
 5. A device in accordance with claim 1wherein the second optical branch comprises a neutral filter.
 6. Adevice in accordance with claim 1 wherein the second optical branchcomprises another cell substantially identical to the cell andcontaining a neutral substance.
 7. A device in accordance with claim 1wherein the first optical branch and the second optical branch eachcomprise at least one optical filter.
 8. A device for opticallymeasuring modifications in a reacting substance contained in atransparent cell, comprising:a first optical branch and a second opticalbranch, an optical director for selectively directing an incident lightbeam through the cell of a first optical branch and through a referencemedium of a second optical branch, a measuring means including a singledetector for measuring light passing through the first or second opticalbranch and producing output signals representing the detected light, acontroller coupled to the output signals including a microprocessor, anelectric power supply providing an electrical supply voltage, aselection means including optical shutters arranged respectively in thefirst optical branch and in the second optical branch, an electricswitching means controlled by the controller, for selectively switchingthe optical shutters to selectively pass the incident light through thefirst and second optical branches, three light sources provided with aspecified light spectrum, a set of three selective filters arrangedrespectively in three optical circuits, a first selective filter fromthe set being centered on a first wavelength corresponding to anisobestic point of the reacting substance, a second selective filterfrom said set being centered on a wavelength in a part of the lightspectrum where the reacting substance is the most sensitive and a thirdselective filter from the set being centered on another part of thelight spectrum where the reacting substance is the least sensitive, saidthree filters being respectively associated with the three lightsources.
 9. A device in accordance with claim 8 wherein the controllerincludes an acquisition unit for acquiring the output signals and aninterface for controlling the selection means.
 10. A device inaccordance with claim 8 further comprising means for measuring anambient temperature and producing a signal representing the ambienttemperature coupled to the controller.
 11. A device in accordance withclaim 8 further comprising means for measuring a supply voltage of eachlamp and producing a signal representing the supply voltage of each lampcoupled to the controller.
 12. A device in accordance with claim 8wherein the second optical branch comprises a neutral filter.
 13. Adevice in accordance with claim 8 wherein the second optical branchcomprises another cell substantially identical to the cell andcontaining a neutral substance.
 14. A device in accordance with claim 8wherein the first optical branch and the second optical branch eachcomprise at least one optical filter.
 15. A method for opticallymeasuring modifications in a reacting substance contained in atransparent cell, with a device having a single light source providedwith an electric supply voltage and specified light spectrum, a firstoptical circuit, a first optical shutter arranged in said first opticalcircuit, a second optical circuit, a second optical shutter arranged insaid second optical circuit, an optical diverter in said optical fibercircuits for diverting incident light from the single light sourcethrough the cell and through a reference medium to an optical node, anoptical separator for directing light from the optical node to threeother optical circuits, a set of three selective filters arrangedrespectively in said three other optical circuits, a first selectivefilter from the set being centered on a first wavelength correspondingto an isobestic point of the reacting substance, a second selectivefilter from said set being centered on a wavelength in a part of thelight spectrum where the reacting substance is the most sensitive and athird selective filter from the set being centered on another part ofthe light spectrum where the reacting substance is the least sensitive,a measuring means for respectively measuring the light emanating fromsaid three other optical circuits, including a set of three detectorsfor respectively detecting light passed by each of the set of threefilters and producing output signals representing the detected light, anelectric power supply for providing the electrical supply voltage, acontroller and an electric switching means controlled by the controllerfor connecting intermittently said three detectors to said controller,for connecting the single light source to the power supply, and forselectively switching said optical shutters comprising:directing lightduring a first stage passing through the cell successively through thethree filters and measuring light intensities outputted from the threefilters; directing light during a second stage passing through thesecond optical branch successively through the three filters andmeasuring the light intensities coming from the three filters; andcombining the light intensity values measured by each detectorrespectively from the first and second stages for determiningcharacteristics of the reacting substance.
 16. A method for opticallymeasuring modifications in a reacting substance contained in atransparent cell, with a device including a first optical branch and asecond optical branch, an optical director for selectively directing anincident light beam through the cell of a first optical branch andthrough a reference medium of a second optical branch, a measuring meansincluding a single detector for measuring light passing through thefirst or second optical branch and producing output signals representingthe detected light, a controller coupled to the output signals includinga microprocessor, an electric power supply providing an electricalsupply voltage, a selection means including optical shutters arrangedrespectively in the first optical branch and in the second opticalbranch, a switching means controlled by the controller, for selectivelyswitching the optical shutters to selectively pass the incident lightthrough the first and second optical branches, three light sourcesprovided with a specified light spectrum, a set of three selectivefilters arranged respectively in three optical circuits, a firstselective filter from the set being centered on a first wavelengthcorresponding to an isobestic point of the reacting substance, a secondselective filter from said set being centered on a wavelength in a partof the light spectrum where the reacting substance is the most sensitiveand a third selective filter from the set being centered on another partof the light spectrum where the reacting substance is the leastsensitive, said three filters being respectively associated with thethree light sources comprising:directing light during a first stagepassing successively through the three filters towards the cellcontaining the reacting substance and measuring respective lightintensity values; directing light during a second stage passingsuccessively through the three filters towards the second optical branchand measuring respective light intensity values; and combining the lightintensity values from the first and second stages for determiningcharacteristics of the reacting substance.